CI: shellcheck: remove apk upgrade command
CI is currently failing at the "apk upgrade" line with the following error. It must be related to the openssl3 transition that is going on in Alpine:
485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: musl-1.2.2-r6: Permission denied
485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: busybox-1.34.0-r3: Permission denied
485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: musl-utils-1.2.2-r6: Permission denied
So let's get rid of it. It shouldn't be needed to install shellcheck anyway.